1. Technical Field of the Invention
The present invention relates generally to an air-fuel ratio control apparatus for internal combustion engines, and more particularly to an improved air-fuel ratio control apparatus which is designed to correct an air-fuel ratio control parameter using outputs from two oxygen sensors arranged upstream and downstream of a catalytic converter, respectively.
2. Background Art
Japanese Patent First Publication No. 62-60941 discloses a two-O.sub.2 sensor system having two oxygen sensors: one being located upstream of a catalytic converter (hereinafter, referred to as an upstream sensor) and the other being located downstream thereof (hereinafter, referred to as a downstream sensor). This conventional system controls an air-fuel ratio around a stoichiometric air-fuel ratio using an output signal from the upstream sensor, and further corrects air-fuel ratio control parameters such as an integration constant, a skip amount, delay time, and a reference voltage, based on an output signal from the downstream sensor for reducing the deterioration of and variations in exhaust emissions due to the variation in characteristic and deterioration with age of the upstream sensor and the variation in engine operation.
The above two-O.sub.2 sensor system monitors an air-fuel ratio downstream of the catalytic converter to determine whether an actual air-fuel ratio falls within a catalyst window (i.e., a region where any of harmful emissions such as NO.sub.X, CO, and HC contained in the exhaust gas is decreased) or not for controlling the actual air-fuel ratio to within the catalyst window. This results in greatly improved emission control. For example, the use of only the upstream sensor will cause the exhaust gas from a specific cylinder to be monitored mainly dependent upon a mounted location of the upstream sensor. This makes it difficult to bring the air-fuel ratio to within the catalyst window. The two-O.sub.2 sensor system can eliminate such a problem to optimize emission control regardless of the variation in engine operation or deterioration of the engine as well as the variation in characteristic of the upstream sensor.
The above prior art system, however, has the drawbacks in that the influence of O.sub.2 storage effects (i.e., a function of accumulating or discharging oxygen) of the catalytic converter increases a time interval between the change in air-fuel ratio upstream of the catalytic converter and the change in air-fuel ratio downstream thereof, resulting in a reduced response rate of the downstream sensor to prolong a control cycle. Additionally, since the air-fuel ratio control parameters are corrected based on the output from the downstream sensor, it is required to delay a correction speed (i.e., a speed at which the control parameters are modified to change an air-fuel ratio) of the air-fuel ratio control parameter for preventing exhaust emissions from being degraded due to the overshoot induced by the reduced response rate of the downstream sensor.
Therefore, because of the prolonged control cycle and the slow control speed, it is difficult or impossible to correct the air-fuel ratio control parameters in transition conditions. The feedback control (hereinafter, referred to as downstream O.sub.2 feedback control) for correcting the air-fuel ratio control parameters through the downstream sensor can be carded out only under steady conditions (e.g., during traveling at a constant speed in an intermediate-high speed range).
For this reason, during traveling conditions other than the steady conditions (e.g., transition conditions), the downstream O.sub.2 feedback control is inhibited, and instead control parameters learned during the downstream O.sub.2 feedback control are used for controlling the air-fuel ratio. This learning is performed in timing where an output from the downstream sensor is reversed between a rich value indicating that an air-fuel ratio is richer than the stoichiometric air-fuel ratio and a lean value indicating that the air-fuel ratio is leaner than the stoichiometric air-fuel ratio, that is, where a difference between an actual air-fuel ratio and the stoichiometric air-fuel ratio may be considered to almost be compensated for. In fact, values of the control parameters upon a rich-to-lean reversal and values of the control parameters upon a lean-to-rich reversal immediately before the rich-to-lean reversal, are averaged to derive learning values.
The above system, however, has suffered from the drawback in that since the learning is not performed unless the output from the downstream sensor is reversed, the optimum control may not be achieved for a long time.
For example, when values of the control parameters upon initiation of the downstream O.sub.2 feedback control are greatly different from optimum values, the reversal of the output from the downstream sensor does not occur for an extended period of time. Thus, during this period, when a gear shift is achieved or the vehicle accelerates or decelerates to bring the vehicle into transition conditions, the downstream O.sub.2 feedback control is prohibited, so that the control parameters are returned back to their respective initial values.
Additionally, a reversal cycle of the output from the downstream sensor is usually as much as several tens to several hundreds of seconds. Therefore, during running in town as well as when the initial values of the control parameters are greatly different from the optimum values, the downstream O.sub.2 feedback control is sometimes opened before the reversal of the output of the downstream sensor occurs. It is, thus, difficult to have the control parameters reach the optimum values due to the length of the control cycle or the delay of the control speed.
Therefore, the above prior art system give rise to problems in that because of the inevitable characteristics of the downstream sensor, exhaust emissions may not be improved for a considerably extended period of time after assembly at the factory or replacement of a battery, initializing the system, and it may be difficult to return the emission control to optimum levels.